This invention relates to a process and an apparatus for liquid-phase extraction of coffee.
It is known that coffee can be extracted on an industrial scale in the manufacture of instant coffee by passing hot water through cells filled with ground roasted coffee (Sivetz - Coffee Processing Technology, Vol. 1, pages 262-263, AVI, 1963).
Extraction is carried out in countercurrent, i.e., the hot water under pressure at a temperature of 150.degree. to 180.degree. C. is introduced into the cell containing the batch of ground roasted coffee which has been most intensively extracted (having undergone N extractions), generally at the bottom of the cell. The liquid extract of this extraction cell is then passed through the extraction cell containing the batch of coffee which has been extracted (N-1) times and so on until the liquid extract passes through the cell which has just been filled with fresh ground roasted coffee.
The final extract leaves this last cell at a temperature of the order of 100.degree. C.
The most intensively extracted coffee is thus subjected to the highest temperature while the fresh coffee is subjected to the lowest temperature.
A distinction is normally drawn between the hot cells, which contain the most intensively extracted coffee, and the cold cells which contain the least intensively extracted coffee.
After each extraction cycle, the cell containing the most intensively extracted coffee is emptied, filled with fresh coffee and, after the cells have been suitably interconnected, another extraction cycle begins.
Although the final extract obtained at the exit of the extraction cell containing the freshest coffee contains only a small quantity of ground coffee particles, fines still being entrained, it is desirable to filter the extract.
Finally, after the filtration phase which eliminates the particles larger than about 1 mm in size, solids, such as polysaccharides, are still present in suspension and are best eliminated to enable a coffee powder which dissolves perfectly without any solids appearing in the cup to be obtained after concentration and freeze-drying or spray-drying of the extract.
The suspended solids are normally eliminated by centrifugation, the sludge obtained then being decanted, the supernatant decantation liquid being reintroduced into the final filtered extract while the solid residue obtained is eliminated.
The main disadvantage of this process is that it produces a sludge which has to be retreated by decantation and which is not easy to handle.
Accordingly, the problem addressed by the present invention was to provide a process for the liquid-phase extraction of coffee which would overcome this disadvantage.